1. Field of the Invention
The present invention relates to a semiconductor device and method of manufacturing such a semiconductor device, and more particularly to a normally-off static induction thyristor and a method of manufacturing such a normally-off static induction thyristor.
2. Description of the Related Art
FIGS. 1 through 3 of the accompanying drawings are illustrative of a conventional static induction thyristor 300 and a method of manufacturing the conventional static induction thyristor 300.
Heretofore, the conventional static induction thyristor 300 has been manufactured as follows:
First, as shown in FIG. 1, a P-type impurity is selectively diffused into one principal surface, i.e., an upper surface, of an N.sup.- substrate 310 to selectively form P.sup.+ gate regions 314.
Then, as shown in FIG. 2, an N.sup.- epitaxial layer 320 is deposited on the N.sup.- substrate 310 by chemical vapor deposition. At this time, P.sup.+ gate regions 314 are also formed in the N.sup.- epitaxial layer 320 by automatic doping.
Thereafter, as shown in FIG. 3, a P layer 312 is formed in a lower surface of the N.sup.- substrate 310 by diffusing an impurity therein, and an N.sup.+ layer 322 is formed in the upper surface of the N.sup.- epitaxial layer 320 by diffusing an impurity therein.
Then, an anode electrode 340 is deposited on a lower surface of the P layer 312, and a cathode electrode 350 is deposited on an upper surface of the N.sup.+ layer 322.
In the static induction thyristor 300 thus formed, the P layer 312 functions as an anode, the N.sup.+ layer 322 as a cathode, both the N.sup.- substrate 310 and the N.sup.- epitaxial layer 320 as an N base 360, and the P.sup.+ gate regions 314 as a gate for controlling an anode current flowing between the anode electrode 340 and the cathode electrode 350.
In order to increase a maximum cutoff current, the conventional static induction thyristor 300 has the P.sup.+ gate regions 314 of a high impurity concentration embedded in the N base 360. To embed the P.sup.+ gate regions 314 in the N base 360, it is necessary to selectively form P.sup.+ gate regions 314 in one principal surface of the N.sup.- substrate 310, as shown in FIG. 1, and thereafter to form the N.sup.- epitaxial layer 320 on the N.sup.- substrate 310 by way of chemical vapor deposition.
Since the N.sup.- epitaxial layer 320 is formed on the N.sup.- substrate 310 after the P.sup.+ gate regions 314 have selectively been formed in the N.sup.- substrate 310, the Nepitaxial layer 320 grown on the P.sup.+ gate regions 314 tend to suffer crystal defects such as stacking faults. Therefore, it has been difficult to produce an N.sup.- epitaxial layer 320 of high quality, and hence it has also been difficult to produce an N base 360 of high quality.
Inasmuch as the impurity of the P.sup.+ gate regions 314 affects the crystallinity of the N.sup.- epitaxial layer 320, a certain limitation is posed on the impurity concentration of the P.sup.+ gate regions 314. As a result, it has not been possible to increase the maximum cutoff current beyond a certain limit.
To manufacture a conventional normally-off static induction thyristor 400 (see FIG. 6 of the accompanying drawings) from the conventional static induction thyristor 300 according to the method of manufacturing the conventional static induction thyristor 300, it is proposed to form P.sup.+ regions 315 (see FIG. 4 of the accompanying drawings) between the P.sup.+ gate regions 314.
FIGS. 4 through 6 of the accompanying drawings are illustrative of a method of manufacturing such a conventional normally-off static induction thyristor 400 according to the method of manufacturing the conventional static induction thyristor 300.
First, as shown in FIG. 4, a P-type impurity is selectively diffused into one principal surface, i.e., an upper surface, of an N.sup.- substrate 310 to selectively form P.sup.+ gate regions 314. A P-type impurity is also selectively diffused into the principal surface of the N.sup.- substrate 310 to form P.sup.+ regions 315 between the P.sup.+ gate regions 314.
Then, as shown in FIG. 5, an N.sup.- epitaxial layer 320 is deposited on the N.sup.- substrate 310 by chemical vapor deposition. At this time, P.sup.+ gate regions 314 and P.sup.+ regions 315 are also formed in the N.sup.- epitaxial layer 320 by automatic doping.
Thereafter, as shown in FIG. 6, a P layer 312 is formed in a lower surface of the N.sup.- substrate 310 by diffusing an impurity therein, and an N.sup.+ layer 322 is formed in the upper surface of the N.sup.- epitaxial layer 320 by diffusing an impurity therein.
Then, an anode electrode 340 is deposited on a lower surface of the P layer 312, and a cathode electrode 350 is deposited on an upper surface of the N.sup.+ layer 322.
In the static induction thyristor 400 thus formed, the P layer 312 functions as an anode, the N.sup.+ layer 322 as a cathode, both the N.sup.- substrate 310 and the N.sup.- epitaxial layer 320 as an N base 360, and the P.sup.+ gate regions 314 as a gate for controlling an anode current flowing between the anode electrode 340 and the cathode electrode 350. Since the P.sup.+ regions 315 are formed between the P.sup.+ gate regions 314, a depletion layer is formed contiguously between the P.sup.+ gate regions 314 even when no bias is applied to the gate. Therefore, the static induction thyristor 400 functions as a normally-off static induction thyristor.
The normally-off static induction thyristor 400 manufactured according to the above process suffers problems, described below, in addition to the problems posed by the manufacture of the conventional static induction thyristor 300.
In the normally-off static induction thyristor 400, not only the P.sup.+ gate regions 314 of high impurity concentration, but also the P.sup.+ regions 315 are embedded in the N base 360. To embed the P.sup.+ gate regions 314 and the P.sup.+ regions 315 in the N base 360, it is necessary to selectively form P.sup.+ gate regions 314 and P.sup.+ regions 315 in one principal surface of the N.sup.- substrate 310, as shown in FIG. 4, and thereafter to form the N.sup.- epitaxial layer 320 on the N.sup.- substrate 310 by way of chemical vapor deposition.
Since the N.sup.- epitaxial layer 320 is formed on the N.sup.- substrate 310 after the P.sup.+ gate regions 314 and the P.sup.+ regions 315 have been formed in the N.sup.- substrate 310, the N.sup.- epitaxial layer 320 grown on the P.sup.+ gate regions 314 and the P+regions 315 tend to suffer crystal defects such as stacking faults. Therefore, it has been difficult to produce an N.sup.- epitaxial layer 320 of high quality not only on the P.sup.+ gate regions 314 but also between the P.sup.+ gate regions 314, and it has also been difficult to produce an N base 360 of high quality.
Inasmuch as the P+ gate regions 314 are formed by diffusing an impurity into one principal surface of the N.sup.- substrate 310, the P.sup.+ gate regions 314 have sound side edges. Consequently, depletion layers extending between the P.sup.+ gate regions 314 do not extend parallel to the direction of the anode current flowing between the anode and the cathode, with the result that a large current cannot be controlled by the gate.